High reliable device for storing heat with reduced manufacturing costs

ABSTRACT

A device that enables storing in a vessel a fluid at high temperature, in which the fluid is under a coverage gas. The tank has a lower concave head and intermediate zone made by cylindrical rings generated by revolution bodies around the central vertical axis, which contains the level of high temperature fluid that is required to store, and an upper closure made up by an upper head with low altitude to diameter ratio to contain the coverage gas, and supported by the intermediate zone.

REFERENCES Patents

-   Elmer W. Rothrock. “Hot Liquid Thermal Energy Storage Tank and     Method”. USA. 4.643.212. Feb. 17, 1987. -   John P. Bell, Joseph M. Bell, Michael S. Bower, Gerard C. Walter.     “Thermal Energy Storage Vessel, Systems, and Methods”. USA     2011/0017196 A1. Jan. 27, 2011. -   Justin Bredar Cutts. “Tank and Pressure Vessel Skirt Thermal     Ratcheting Prevention Device”. USA 2013/0087569 A1. Apr. 11, 2013. -   John P. Bell, Joseph M. Bell, Michael S. Bower, Gerard C. Walter.     “Thermal Energy Storage Vessel, Systems, and Methods”. USA     2014/0060771 A1. Mar. 6, 2014.

Other Publications

-   World Energy Council. “World Energy Resources E-Storage—2016”. World     Energy Council—World Energy Resources, 2016. -   Xavier Py, Najim Sadiki, Regis Olives, Vincent Goetzand, Quentin     Falcoz. “Thermal energy storage for CSP (Concentrating Solar     Power)”. 5th course of the MRS-EMRS “Materials for Energy and     Sustainability” and 3^(rd) course of the “EPS-SIF International     School of Energy”. EDP Sciences-SIF, 2017. -   Thomas Bauer, Nils Breidenbach, Nicole Pfleger, Marcus Eck.     “Overview of Molten Salt Storage Systems and Material Development     for Solar Thermal Power Plants”. World Renewable Energy Forum-2012.     May 2012. -   Patricia Kuntz Falcone. “A Handbook for Solar Central Receiver     Design”. Sandia National Laboratories. SAND 86-8009. December 1986. -   C. J. Winter, R. R. Sizmann, L. L. Vant Hull. “Solar Power Plants,     Fundamentals, Technology, Systems, Economics”. Springer Verlag 1991. -   Craig E. Tyner, J. Paul Sutherland, and William R. Gould Jr. “Solar     Two: A Molten Salt Power Tower Demonstration”. Sandia National     Laboratories. SAND 95-1878C. August 1995 -   Alexis B. Zavoico. “Solar Power Tower Design Basis Document,     Revision 0”. Sandia National Laboratories. SAND2001-2100. July 2001. -   Robert Moore, Milton Vernon, Clifford K. Ho, Nathan P. Siegel,     Gregory J. Kolb. “Design Considerations for Concentrating Solar     Power Tower Systems Employing Molten Salt”. Sandia National     Laboratories. SAND2010-6978. September 2010. -   Phil Taylor. “SOLAR: Nev. plant solves quandary of how to store     sunshine”. EENEWS. Mar. 29, 2016. -   Juan Ignacio Burgaleta, Santiago Arias, Diego Ramirez. “Gemasolar,     the first tower thermosolar commercial plant with molten salt     storage”. Proceeding of the International Solar PEACES Conference.     Marrakech, Morocco, September 2012. -   Henry Brean. “Salt leak shuts down first-of-its-kind solar plant     near Tonopah”. Las Vegas Review Journal. December 2016. -   Sener. “Overhaul en las plantas de Torresol Energy”. Sener Noticias.     N^(o) 53. June 2017. -   James E. Pacheco. “Final Test and Evaluation Results from the Solar     Two Project”. Sandia National Laboratories. SAND2002-0120. January     2002. -   Bruce Kelly. “Advanced Thermal Storage for Central Receivers with     Supercritical Coolants”. Abengoa Solar. Grant DE-FG36-08GO18149.     June 2010.

SUMMARY

In this invention is presented a new device that enables storing heat in a vessel, by enclosing a fluid at high temperature, using a design consisting of a concave lower head and intermediate body supported from the top, which enables free radial and axial dilatation, contains a minimum volume of hot fluid in a central extension outwards as a tank heel, have very low manufacturing costs and weight, having a second upper head designed to close the ullage gas.

TECHNICAL FIELD OF THE INVENTION

The main object of this invention submitted for an invention patent is a device for storing heat with high reliability at high temperature and low manufacturing cost.

More specifically, this invention consists of a high reliable device for storing heat by enclosing a fluid at high temperature, with an inexpensive construction that enables, with low investment cost, its use for storing energy as a molten salt at high temperature and atmospheric pressure.

Prior Art and Problems to Solve

For many years fluids have been stored in vessels and tanks at ambient pressure and temperature, but recently has been and increasing interest in storing thermal energy as a molten salts at high temperature. As analyzed by the World Energy Council in the year 2016, when big amounts of energy, in the order of tenths to hundredths of megawatts per hour, are currently required to be stored for many hours with high efficiency, only the thermal energy storage by means of molten salts is available, since the other large-scale alternative option, i.e. the use of pumping hydroelectric facilities, only can used under very specific and limited geographic and hydrological conditions, with limited efficiencies.

As explained by Xavier Py et al. in the year 2017, the fact that sun and wind availability is intermittent and unmatched to consumers' needs has strongly increased interest in concentrated solar power technologies with central receivers, since in these plants it is significantly easy, inexpensive and efficient to directly store in a “two-tank system” the heat derived from the concentrated solar power receiver, so that the heat can be used for electric power generation by means of a steam turbine according to the operator's decision and to the heat that plant was able to store during the diurnal period. In these plants, the molten salt enters, from a first tank of molten salt at low temperature, into a receiver that gets sunlight as concentrated with mirrors that are oriented in a controlled way, so that the molten salt gets out at high temperature and comes to be stored in a second tank of hot molten salt. Under the same reference is explained that in the recently inaugurated Gemasolar (Spain, 2011) and Crescent Dunes (USA, 2015) plants up to 20 and 110 megawatts of electric power can be produced, respectively, with 15 to 6 hour storage headacity, which enables to significantly reduce intermittence and deliver the stored thermal energy according to the electric power demand.

For this reason, as explained by Bauer et al. in the year 2012, the two-tank molten salt system is the only system commercially available in large thermal storage headacities for concentrated solar power plants. The success and consolidation of this system resulted from developing and building over a number of years different technology prototype-plants to store the generated solar power in the central receiver.

As described by Falcone in a design handbook for this type of solar power plants published in the year 1986, several methods for storing large-scale thermal solar power were tested in the '80s. In the Themis plant, France, and in the CESA-I plant, Spain, were developed the first prototype plants of 2.5 and 1 electric megawatt/hour, respectively, with a storage headability of 12 and 3 electric megawatt/hour each, respectively, using the two-tank molten salt design for the first time. In the same time period, the first large-scale test was performed in the USA for two-tank molten salt storage, with a storage headacity of 7 thermal megawatt hour, in the CRTF facility.

The feasibility and competitiveness of storing large volumes of molten salt at very high temperature depend, to a large extent, on the feasibility of being able to build tanks headable of holding the whole of that volume with low heat loss, implying an investment compatible with electric power generation, and with a reliability that enables failure-free operation over long periods of time.

As detailed by Winter et al. in the year 1991, that Themis and CESA-I plants were using horizontal cylindrical tanks or vessels with spherical heads, while the CRTF facility used vertical cylindrical tanks with a flat bottom that is supported on a refrigerated concrete-made foundation.

According to Falcone's discussion, the two hot salt storage tanks can be designed in three alternative designs, i.e. flat-bottomed vertical cylindrical tanks with external thermal insulation, flat-bottomed vertical cylindrical tanks with internal thermal insulation for the hot tank and external thermal insulation for the cold tank, or several horizontal cylinders for both fluids. In the same discussion, Falcone mentions that the spherical tanks were considered in some early designs, but they were subsequently discarded as more expensive than the alternative options above mentioned. A similar discarding of spherical tanks, as more expensive, was also mentioned in 1987 by Rothrock, in a letter patent for reducing structural solicitation affecting vertical cylindrical tanks.

Both in Rothrock's letter patent and in the Falcone's design handbook, it is explained that the vertical cylindrical tanks undergo strong thermal-mechanic solicitation caused by the creep and stress exerted on the joining of the flat bottom with the lateral cylindrical walls, and the efforts and degradation undergone by the lower base as said lower base has to slide with regard to its supporting base due to the periodical tension and dilatation cycles. These problems are more serious for the tanks subject to higher temperature. In the current commercial facilities, the tank in which the liquid salt is stored at lower temperature before getting into the receiver, also called “cold tank”, is kept at a temperature of around 290° Celsius, while the tank in which the salt is stored after its having been heated in the solar receiver, also called “hot tank”, is kept at a temperature of around 565° Celsius.

Due to these problems encountered with the vertical cylindrical tanks that are supported from their lower base, Falcone et al. mention the alternative option of flat-bottomed tanks supported from their top or by means of lateral columns, as Winter et al. mention the possibility of conical tanks with very narrow angle that are supported from their external perimeter.

After several tests and configurations performed in the '90s and at the beginning of year 2000, it was established that the best system, on account of its renderings, efficiency and costs, was the molten salt storage directly in two flat-bottomed vertical cylindrical tanks. Therefore, as detailed by Tyner et al. in the year 1995, the storage system of the Solar One's prototype plant, which used heat exchangers and a single rock and sand tank, was replaced. In this plant, which was renamed as “Solar Two”, heat was being stored directly by means of a system consisting of two flat-bottomed vertical cylindrical tanks, supported by/on an insulating base and refrigerated concrete.

As from its successful operation, for more than two years, of Solar Two prototype plant in the USA at the end of the '90s, the plant was closed and dismantled, having set the Design Bases for the current concentrated solar power plants, with the more successful and currently operating central receivers, as the aforesaid Gemasolar plants.

These Design Bases were set forth in the year 2001 by Zavoico, in the Design Basis Report published by Sandia National Laboratories, USA. This report includes the Design Bases for heat storage at high temperature by means of molten salts, using the design of two flat-bottomed vertical cylindrical tanks with external thermal insulation, as supported on an insulating sliding lower base and on refrigerated concrete. Similar thereto is the design described by Moore et al.'s subsequent report in 2010, published by the same laboratories.

Current plants follows the Sandia National Laboratories' Design Basis guidelines for the heat storage systems, through the horizontal two-tank system, as available in the descriptions provided by Taylor and Burgaleta in the years 2016 and 2012, respectively.

A more detailed analysis, however, shows that the successful operation of the tanks in the Solar Two plant does not match in such a direct way with the Crescent Dunes and Gemasolar tanks operation. As described by Brean in the year 2016, Crescent Dunes had to stop operating for a number of months in order to repair a hot tank failure upon approximately one year's operation, while, as outlined by Sener in the year 2017, the Gemasolar plant had to fully replace a hot salt tank before 6 years operation. These times of operation between failures should be compared with the classical purposes of a commercial plant, in which, for amortizing significant headital costs it is usually required that the plant systems and components can operate for long periods of time, typically of 20 to 25 years.

As detailed by Pacheco in the year 2002, the Solar Two tanks were of 11.6 meter diameter and 8.4 meter height, could store 106.5 thermal megawatt hour, and were tested for an approximate period of 15,000 hours each tank. In comparison with those tanks and that operation background, the Gemasolar tanks are of 23 diameter and 11 meter height, and can store 670 thermal megawatt hour, and their hot tank was replaced upon approximately 50,000 operation hours, while the Crescent Dunes tanks are of 43 meter diameter, 12 meter height, and can store 2700 thermal megawatt hour, and major repairs had be made on the hot tank upon approximately 10,000 operation hours.

It clearly results, therefore, that extrapolation to larger dimensions and longer operation times, as compared with the values tested on the Solar Two tanks, required for a commercial plant has generated a major problem to the commercial development and the reliable operation of concentrated solar plants fitted up with heat-storage central receivers.

As these problems are not fully unexpected, proposed devices can be found, e.g. in Rothrock's letters patent of year 1987, Bell et al.'s letters patent of years 2011 and 2014, and Cutts' letters patent of year 2013, which intend to do away with the tension and deformation challenges implied by the flat-bottomed support with a hot fluid at such a high temperature, with daily changes in level and temperatures. Thus it can also be understood that at the start of this technology other alternative options have been evaluated as compared with the bottom-supported vertical cylindrical tanks, such as the proposals above mentioned, i.e. Falcone's and Winter's in the years 1986 and 1991 respectively.

A strong deviation that can explain tank failure and replacement in the commercial plants is that, as mentioned in the Zavoico's Design Bases, the hot salt storage tanks are specified under standard API 650, and this standard sets an operation limit of up to 260° Celsius for refrigerated tanks, which is much lower than the 565° Celsius for hot salt tank. At a temperature significantly higher than the standard limit value, failure risk is higher and reliability is low.

Additionally another problem is encountered, overlooked in the costs of concentrated solar plants with central receivers, as directly incorporated in the Design Bases without an impact critical analysis, i.e. flat-bottomed vertical cylindrical tanks are assumed to be the only feasible option for heat storage.

The electric engines of the molten salt pumps are fitted outside and above each tank, while the pump driver is submerged near the bottom of the tank by means of a long shaft and vertical pipe that drives and carry the salt towards an output nozzle located near the electrical engine, above and outside the tank. The cost of the molten salt pumping system is thus simplified, made reliable, and reduced, making use of the advantage that the tanks are under atmospheric pressure.

The submerged pumps require a minimum submergence or fluid level above their suction inlet, so that they do not generate vorticity that could drag air as the salt enters into the pump inlet, and a minimum clearance below their suction inlet so that they do not generate any restriction for salt circulation at said pump inlet. According to the value recommended in the Design Bases published by Sandia National Laboratories, the minimum salt height for each tank should be equal to 1 meter of molten salt or higher. This minimum depth of molten salt, or molten salt heel or tank bottom, generates two economic effects that significantly penalize the plant cost.

In a first effect, to the salt volume that each tank should be headable of storing to contain the heat that is required to store, is added the salt volume from the heel that is required to operate this tank plus the salt heel volume from the other tank, because each tank should be able to store whole volume of the molten salt in both tanks, in the event of drying the other tank for maintenance. As the volume of this tank heel neither can nor should be driven by the pump, it is not an integral part of the molten salt stock that is utilized for heat storage purposes. Considering that each tank should thus be headable of storing a salt volume that is twice the salt heel of each tank, and that the height of a tank can typically range from 10 to 14 meters, these two meters approximately represent an increase in the tank volume and weight of approximately 20% as compared with the volume strictly required for heat storage purposes, which in principle is consistent with a similar percentage in the increase in the cost of each tank.

In a second effect, the total salt inventory necessary to operate the plant also requires, besides the salt mass required to contain the heat required to store, the operating stock of the piping and components of the molten salt circuits, and the inactive mass of both heels of the tank. This implies an increase in the total salt inventory, also in the order of 20%. If it is considered that one of the main headital costs for this type of plants is the cost of the salt inventory, this main cost is also increased in approximately 20%.

Taking as reference the cost ratios of the different components of a commercial plant shown by Kelly in the year 2010, the total direct costs of a commercial plant increase by 1.5% due to both effects, with 0.4% thereof relating to the higher cost of the tanks and 1.1% thereof relating to the higher cost of the salt. Another way to represent on an economic basis the penalty implied by both effects is also obtained by comparing the additional cost generated by the cost of the tanks. In this case, it results that the increase in costs is equivalent to 90% of the cost of the two tanks.

The flat-bottomed vertical cylindrical tanks, which reduce the manufacturing cost by transfer the weight to the foundations and using single curvature metal sheets, has the disadvantage that requires more material weight and welding thickness, increasing the manufacturing costs, for those part of the tank with less height by the increase of the hydrostatic pressure, compared with curved casks which requires the less material weight and welding thickness.

There thus exist the actual requirement to have an inexpensive device, and competitive with horizontal cylindrical tanks, headable of storing a molten salt heat with lower failure rate and higher reliability, significantly reducing the direct additional costs generated by the tank heel demands from the molten salt pumps and significantly reducing the manufacturing costs of the tanks. ACA

Short Description of the Invention

Considering the problems described with regard to the current devices for storing heat in molten salts at high temperature, this invention has been conceived to solve those problems.

In other words, this invention is intended to have a device that enables to store heat by enclosing a fluid at high temperature, with inexpensive construction, allowing its application for energy accumulation in the form of molten salt at high temperature and atmospheric pressure, with high reliability and low risk, significantly reducing the costs implied by the tank heel demands from the molten salt pumps.

The device can be applied to store other fluids at high temperature, as well as those fluids that can require storage at higher working pressures.

For accomplishing the purpose proposed, the device consists of a tank or vessel made-up of the joining two curved shells, the axis of revolution of which is vertically oriented, i.e. the lower shell is a curved head welded to an upper shell build by rings of conical sections, in which the two curved shells contains the hot fluid and is supported with ground clearance from its upper portion, so that the vessel is free from thermally expand to the supporting plane.

The vessel ends on the top by an upper head to contains the ullage gas.

For lowering the tank weight and manufacturing cost, the device could use a concave lower head of ellipsoidal shape, in which the ellipsoid height to diameter ratio is higher than or equal to two, and it is designed according to the design pressure generated by the weight of the hydrostatic column of the stored fluid and the height of the supporting upper zone. The device can also use other concave shapes, such as a torispherical head, according to the production costs of one or another head.

In order to lower the manufacturing cost, the vessel could ends on the top in the central zone by a flat vessel head connected to a conical section build by facetted planes or by a ring of cylindrical truncated cone.

In order to lower the tank weight and the manufacturing cost, the device uses an upper head with height to diameter ratio higher than or equal to four, which can be of torispherical or ellipsoidal shape, designed to withstand its own weight and the pressure generated by the slight overpressure exerted by the ullage gas, plus the expectable overpressure exerted during transitory events and the opening pressure of the ullage gas relief valves.

For minimizing the economic penalty involved in using fluid impelling pumps with their engine fitted outside and above the tank and their driver submerged near the bottom, the lower head could have a central downward extension in its center, which ends with a concave lower head, the depth with regard to the beginning of the extension being similar to the height required as heel for pumping the coolant, and an internal diameter that enables suction without generating restrictions to the fluid getting into the pump. This extension exerts the effect of the so-called pumping wells and for its small size, weight and simple construction it is of low cost body, that also requires a much smaller quantity of molten salt than the molten salt quantity required for heat storing.

The union between the lower head and the central downward extension could be made with a smooth transition revolution body to reduce the local stress.

As there is an technological limit with regard to the reachable highest depth for the molten salt pumps currently available, in which the engine is fitted outside and above the tank and the driver is submerged near the bottom, the device uses an ellipsoidal shape for the lower head, of height to diameter ratio equal to two, in the case the pump required by said tank is compatible with the depth technologic limit of the available pumps, i.e. in that case, the device uses a semispherical head, cylindrical truncated cone, flat head or faceted cylindrical truncated cone.

When the pump required by a tank is within the depth technologic limit of the pumps currently available, the device uses an upper head of inverted type to minimize the distance the distance between the molten salt upper limit level and the center of the upper head. The inverted type upper head could have a concave curve head or flat head at the center, with a smooth transition revolution body to the external zone of the upper head which had a higher height than the central height. The external zone of the upper head could combine toroidals, conical sections, planes or faceted conical sections shapes.

Comparing the amount of steel required by a sphere to contain a given volume of fluid at atmospheric pressure, it is noticed that a sphere requires approximately four times the mass for two times the volume of a hemisphere. Even though they would seem geometrically similar, with the same diameter, the upper portion increases by two times the design pressure, by two times the weight of the hydrostatic column, which increases by four times the weight of the metal sphere, containing only two times the volume stored with regard to the hemisphere.

Therefore, this lower efficiency on material use of the sphere could be the reason that explain why in the past several authors discard spherical tanks compared with vertical cylindrical tanks. Nevertheless, as derived from the foregoing explanation, this discarding does not apply to a device containing hot fluid with the shape proposed in this invention.

Considering that, besides, the proposed device significantly reduces the salt weight and tank volume penalty generated in the pumping heel demand for operating the tank when it is at minimum level for flat-bottomed vertical cylindrical tanks, advantage of this device is even clearer.

From the functional and economic viewpoint, the proposed device requires less weight of vessel than the horizontal cylindrical tanks

The horizontal cylindrical tanks, requiring higher thickness at the bottom, requires higher costs in materials, transport, construction, joints preparation, and welding compared with the proposed device.

Besides, on account of its geometry, supporting, materials, loads, corrosion, and temperature, this device is fully admissible with the design under ASME VIII Code, as all of these parameters are included in those available in said Code. For this reason, and considering the high reliability and low failure rate of the vessels designed and constructed according to the ASME Code, failure risk and low reliability are significantly reduced in the proposed device.

In addition, the vessel support and loads transfer from to the foundation could be performed with well known supports design alternatives for ASME Vessels, either using static or sliding commercial available supports technologies, at loads and temperatures required for thermal heat storage.

SHORT DESCRIPTION OF THE DRAWINGS

To better understand this invention, we provide below a detailed description on the basis of the following figures, which serve the sole purpose of illustrating the preferred way to execute this patent and not to represent a limit to the invention:

FIG. 1 is a schematic section of the flat-bottomed vertical cylindrical tanks used by commercial plants for storing the molten salts.

FIG. 2 is a schematic section of the proposed device, made up of the joining of two concave heads that have their axes of revolution vertically oriented, the lower head is a hemisphere or ellipsoid of height to diameter ratio equal to 2, the proposed device has a pumping well, made up by a central downward extension the bottom of which is a lower curved head, the whole vessel is supported from the middle of the lower head, and the higher head contains the ullage gas.

FIG. 3 is a schematic section of the proposed device, in which the molten salt is contained in 3 cylindrical bodies, a ellipsoidal head at the bottom, a conical section at the middle with the connection to the support system, and a cylindrical section at the top. The ullage gas is contained by an upper head build by combining an external conical section with a central flat heat.

FIG. 4 is a schematic section of the proposed device, in which the molten salt is contained in 4 cylindrical bodies, a lower ellipsoidal head at the bottom, a conical section over the bottom head with another revolution body on the top that provides a smooth transition to the support system, and a conical section on the top. The ullage gas is contained by an inverted type upper head build by an external toroidal section and an internal concave head.

FIG. 5 is a schematic section of the proposed device, with a pumping well build by a central downward extension at the center of the lower ellipsoidal head, a conical section at the middle with the connection to the support system, and a cylindrical section at the top. The ullage gas is contained by an upper head build by combining an external conical section with a central flat heat.

FIG. 6 is a external view of the proposed device, where the upper head to contain the ullage gas is build by central flat head, and an external conical section made by an arrange of faceted inclined planes.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE OF EMBODIMENT

In this invention a new device is presented that enables to store heat by enclosing a fluid at high temperature, with an inexpensive construction, enabling its application for energy accumulation in the form of molten salt at high temperature.

FIG. 1 is a schematic section of the vertical cylindrical tanks, typical from the previous state-of-the-art, used by commercial plants for storing heat in molten salts. The tank is made up of a first lower level of a cylindrical body (110), with a vertical axle, of a thickness that enables it to bear hydrostatic pressure and thermal-mechanic stresses, which is then continued by other straight cylindrical bodies (114) and (116) that can be of lesser thickness as the height increases and the hydrostatic pressure and the thermal-static stresses decrease. In this case, the tank is vertically made up by three thicknesses, only for illustrative purposes, but a commercial tank can be made up of a higher number of successive stages of different thickness. The upper head, with the function of containing the ullage gas at slight overpressure, is made up of the curved head (117), while the flat bottom (118) constitutes the watertight base of the tank. The flat bottom (118) transfer the loads and can accommodate its dimensional changes by lying on a sliding surface area (120), which is thermally insulated by the body (122), which lies on the base and on the refrigerated concrete foundations (124), by means of a cooling system that is not shown in the figure, which transmits the load to the surrounding ground (126). The tank is thermally insulated from the external air by means of the cover, (130) and (132). There is schematically shown the vertical pipe (140) of the molten salt pump, the inlet nozzle and impeller (142) of which are located at the lower end of the vertical pipe (140), while the electric motor is located in the upper section (144), together with the molten salt outlet nozzle and pipe (146). Between the lower surface area (150) of the tank bottom (118) and the line (152) of molten salt minimum level is the volume (153) of molten salt, or tank heel, required by the impeller and the inlet nozzle (142) of the salt pump for appropriate operation. There is schematically shown as volume (154) the tank heel of another tank, not shown in the figure, which is required to be stored by the tank that is shown in the figure, for those cases where the contents of the second tank should be emptied for repairs and maintenance purposes. This second volume of tank heel has the upper limit marked-out by the level (155). Above this level (155) is schematically shown the tank volume available to store the heat (156), which has as its upper limit the maximum level of salt in the tank (157). Above this level (157) and the upper head (117), is the coverage or ullage gas (158), which is usually made by dry and clean air, at slight overpressure, as compared to external pressure, to reduce the intake of air impurity and humidity into the salt. In this figure, even though dimensions are exaggerated for illustrative purposes, it can be clearly seen how to the molten salt volume required for heat storage (156) are added the molten salt volumes (153) and (154) generated by the tank heel criterion of a flat-bottomed tank, for defining the total tank and salt volumes required to store heat with this design.

FIG. 2 is a schematic section of the proposed device, made up of a lower concave head (200), which is schematically shown as a hemisphere and has in its lower central portion a downward extension made up of a cylindrical body (202) with a lower cover made up of a concave head (204). The lower central extensions (202) and (204) contain the tank heel (210) or pumping well, and the molten salt level of which is schematically shown as the line (212). Should it be required to transfer, to the tank shown in FIG. 2, the tank heel from the second tank, not shown in the figure but of similar shape than the tank shown in FIG. 2, this salt volume (214) also results from a very low upper limit (216), given the concave shape of the head (200). Above these two heel levels is the molten salt volume required for storing the heat (217), the upper limit of which is schematically shown as the line (218). Above the line of maximum salt level (218) is the ullage gas (220), which is enclosed by the upper concave head (222). In the figure, the lower head (200) is schematically shown as of thickness significantly higher than the upper head (222), since the first has as design pressure the pressure of the hydrostatic column of molten salt, while the second has as design pressure the slight overpressure required to secure that no air impurities and humidity enters into the ullage gas and be able to support its own weight and any mechanical device attached to the vessel head. Both the lower head (200), including its tank heel, (202) and (204), and the upper head (222) are covered by a thermal insulation (230) (232) (234) and (236) to reduce the thermal losses to the external air. The lower head (200) is supported by a support system (240) made up of sliding supports (242) and (244) typical for pressure vessels, selected for bearing the whole weight of all the molten salt and the vessel, at the molten salt temperature. A lower structure (250) and a local thermal insulation (252) enable to transfer the loads to a supporting structure (254) at ambient temperature, which finally transmits the load to the ground (256). Since the sliding supports (242) and (244) enable to freely dilate the tank in the radial direction, while the bottom of the tank heel (204) and its thermal insulation (234) have a given clearance from ground level (256), the proposed device enables the tank to freely dilate in an axial and radial directions. These characteristics allow to absorb the dilatations, with no mechanic stress taking place, in the tank or its supports, ensuring that the loads are within the load range of the vessel support standards. The device also requires a low metal weight for the whole tank assembly, since a hemisphere requires approximately half the weight, per contained volume, than a full sphere, provided the fluid is subject to an upper pressure similar to atmospheric pressure, because in that case design pressure is mainly given by hydrostatic pressure. For illustrative purposes, and as the tank heel, (202) and (204), is centered on the bottom of the lower head (200), the pump extracting salt from the tank is schematically shown as a single pump (260), but in the specific designs there can be present several pumps, not shown in the figure but all of them submerged into the tank heel, (202) and (204). It can be qualitatively noticed how the salt volume of the two tank heels of this device (210) and (214), is comparatively much smaller than the volume of the tank heels (153) and (154) of the flat-bottomed cylindrical tanks, like those shown in FIG. 1, commercially used at present. The shape of the lower head (200) that contains the molten salt above the lower heal (210) and (214) and below the level line (218) required a bi-dimensional shaping of the metal sheets at all the submerged surface of the head (200), which will increase the cost of the curved metal sheets, joint machining, alignment and welding, in such a way that even considering the concave shape will require a small thickness for the metal sheets, the manufacturing cost could jeopardize the final cost of the vessel build by a single main concave head.

FIG. 3 is a schematic section of the proposed device even cheaper to build than the design alternative of FIG. 2, to store the molten salt (300), with is fluid maximum level line (302) between the lower line (304) and the upper line (306). Below the lower line (304) is the concave lower head (310), while above the lower line (304) and the upper line (306) the intermediate body of the vessel is made by 2 revolution rings (320) and (325), to simplify the explanation, but the intermediate body of the vessel could be made from a single revolution ring to several revolution rings to reduce the vessel manufacturing costs. The upper revolution ring (325) is a vertical cylinder with reduce manufacturing cost because the design pressure is given by the hydrostatic pressure produce the difference in height between the higher level line (302) and the height of the bottom end of the cylinder (325). The lower revolution ring (320) is made by a conical section to have a reduced manufacturing costs in addition to give structural support to the union between the vessel and the support structure and withstand the hydrostatic pressure give by the height difference between the higher level line (302) and the height of the bottom end of the cylindrical cone (320). The revolution rings (320) and (325) of the intermediate body have low manufacturing cost because have low thickness and requires shaping in a single direction, applying only to the lower head the two dimensional shaping required for a concave head, and where the thickness is keeping low due to the inherent resistance of concave bodies subject to internal pressure produced by the higher hydrostatic pressure, and without needing to be able to support the concentrated loads produced by the support system. The vessel support system (330) is connected with one of the rings of the revolution ring of the intermediate zone, in this case shown as the ring (320), to transfer the load to the civil structure (335), that finally transfer the load to the ground (340). The ullage gas (350), above the upper line (306) is enclosed by an upper head build by an external conical section (360) and a central flat head (365). This upper head have low manufacturing cost by the inherent nature of the shape to support its own weight, the weight of the molten salt pumps (367) and (368), and the very small overpressure required in the ullage gas to compensate the any small leakages in the upper vessel head. All the vessel is covered by a thermal insulator (370) (372) (374) and (376) to reduce the thermal losses to the ambient air. The supporting device (330) has its own thermal insulation (378) to ensure that the thermal stress are compatible with the union between the supporting device (330) and the civil structure (335).

FIG. 4 is a schematic section of the proposed device to store the molten salt (400), with is fluid maximum level line (402) between the lower line (404) and the upper line (406). Below the lower line (404) is the concave lower head (410), while above the lower line (404) and the upper line (406) the intermediate body of the vessel is made by 2 revolution rings (420) and (425), with a revolution body (427) that connects the revolution rings (420) and (425) with the supporting structure (430). The intermediate body of the vessel is made by two revolution rings to simplify the explanation, but the intermediate body of the vessel could be made from a single revolution ring to several revolution rings to reduce the vessel manufacturing costs. The support structure (430) transfer the loads to the civil structure (435), that finally transfer the load to the ground (440). The ullage gas (450), above the upper line (406) is enclosed by an upper head build by an external toroidal section (460) and a central concave head (465). This upper head have low manufacturing cost by the inherent nature of the shape to support its own weight, the weight of the molten salt pumps (467) and (468), and the very small overpressure required in the ullage gas to compensate the any small leakages in the upper vessel head. All the vessel is covered by a thermal insulator (470) (472) (474) and (476) to reduce the thermal losses to the ambient air. The supporting device (430) has its own thermal insulation (478) to ensure that the thermal stress are compatible with the union between the supporting device (430) and the civil structure (435).

FIG. 5 is a schematic section of the proposed device to store the molten salt (500), with is fluid maximum level line (502) between the lower line (504) and the upper line (506). Below the lower line (504) is the concave lower head (510), while above the lower line (504) and the upper line (506) the intermediate body of the vessel is made by 2 revolution rings (520) and (525). The intermediate body of the vessel is made by two revolution rings to simplify the explanation, but the intermediate body of the vessel could be made from a single revolution ring to several revolution rings to reduce the vessel manufacturing costs. The upper revolution ring (525) is a vertical cylinder with reduce manufacturing cost because its design pressure is given by the hydrostatic pressure produce by the difference in height between the higher level line (302) and the height of the bottom end of the cylinder (325). The lower revolution ring (520) is made by a conical section to have a reduced manufacturing costs in addition to give structural support to the union between the vessel and the support structure and withstand the hydrostatic pressure give by the height difference between the higher level line (502) and the height of the bottom end of the cylindrical cone (520). The revolution rings (520) and (525) of the intermediate body have low manufacturing cost because have low thickness and requires shaping in a single direction, applying only to the lower head the two dimensional shaping required for a concave head, and where the thickness is keeping low due to the inherent resistance of concave bodies subject to internal pressure produced by the higher hydrostatic pressure and without needing to support the concentrated loads produced by the support system. Below the bottom line (580) the lower head (510) ended y a new volume (582) of molten salt that makes the tank heel or pumping well, build by a cylindrical ring (585) and a small lower head (590), with its molten salt level given by the line (580). The vessel support system (530) is connected with one of the rings of the revolution ring of the intermediate zone, in this case shown as the ring (520), to transfer the load to the civil structure (535), that finally transfer the load to the ground (540). The ullage gas (550), above the upper line (506) is enclosed by an upper head build by an external conical section (560) and a central flat head (565). This upper head have low manufacturing cost by the inherent nature of the shape to support its own weight, the weight of the molten salt pumps (567) and (568), and the very small overpressure required in the ullage gas to compensate the any small leakages in the upper vessel head. All the vessel is covered by a thermal insulator (570) (572) (574) (576) (592) and (594) to reduce the thermal losses to the ambient air. The supporting device (530) has its own thermal insulation (578) to ensure that the thermal stress are compatible with the union between the supporting device (530) and the civil structure (535).

FIG. 6 is a external view of the proposed device, where the thermal insulations and surrounding ground is not shown, in which the intermediate body is made by a upper vertical cylinder (600) and a lower cylindrical cone (610), connected to a support skirt (620), and the lower body is made by the concave head (625). The support skirt (620) transfer the loads to the civil structure (630), made by pre casted-bodies, which transfer the loads to the pre-casted foundations (635). The ullage gas is closed by the upper vessel head made by a short cylindrical vessel head (640), and azimuthally arranged of flat planes to resemble a conical section, as the flat plane (650), and a central flat head (660) at the center and the top. Pumps and other opening to the upper vessel head are arranged in the central flat head (660) and also in several flat sheets, as the flat sheet (650) but without any specific identification line, because are only shown as a schematic example of the loads that needs to be considering when the upper head is designed.

Following, 117 claims are provided on pages 21 to 29: 

1. A device that enables storing in a vessel a fluid at high temperature, in which the fluid is under a coverage gas, wherein said vessel is made up by a lower concave head and intermediate zone made by cylindrical rings generated by revolution bodies around the central vertical axis, which contains the level of high temperature fluid that is required to store, and an upper closure made up by an upper head with low altitude to diameter ratio to contain the coverage gas, and supported by the intermediate zone.
 2. A device that enables storing in a vessel a fluid at high temperature, according to claim 1, wherein the intermediate zone is made by conical sections with an increasing angle between the vertical central axis of the vessel and the conical face, when the cylindrical ring is lower than upper cylindrical ring.
 3. A device that enables storing in a vessel a fluid at high temperature, according to claim 2, wherein the intermediate zone is made by only one vertical cylinder with low altitude to diameter ration.
 4. A device that enables storing in a vessel a fluid at high temperature, according to claim 2, wherein the upper cylindrical ring of the intermediate zone is made by one vertical cylinder with low altitude to diameter ration, and the lower cylindrical rings of the intermediate zone are conical sections with an increasing angle between the vertical central axis of the vessel and the conical face, when the cylindrical ring is lower than upper cylindrical ring.
 5. A device that enables storing in a vessel a fluid at high temperature, according to claim 2, wherein the intermediate zone is made by only one conical section with an angle between the vertical central axis of the vessel and the conical face that is different to zero.
 6. A device that enables storing in a vessel a fluid at high temperature, according to claim 1, wherein one of the cylindrical ring of the intermediate zone is made by the revolution body created from a conical section of the vessel and a section of the supporting structure.
 7. A device that enables storing in a vessel a fluid at high temperature, according to claim 6, wherein the upper cylindrical ring of the intermediate zone is made by one vertical cylinder with low altitude to diameter ration, and the cylindrical ring immediately below the cylindrical ring is made by the revolution body created from a conical section of the vessel and a section of the supporting structure.
 8. A device that enables storing in a vessel a fluid at high temperature, according to claim 6, wherein the upper cylindrical ring of the intermediate zone is made by one vertical cylinder with low altitude to diameter ration and also goes downward as part of the supporting structure.
 9. A device that enables storing in a vessel a fluid at high temperature, according to claim 6, wherein one lower cylindrical ring of the intermediate zone is made by a conical section that also goes downward as part of the supporting structure.
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 16. A device that enables storing in a vessel a fluid at high temperature, according to claim 4, wherein the upper head is made by a concave head respective to an exit vector to the vessel, with low height to diameter ration.
 17. A device that enables storing in a vessel a fluid at high temperature, according to claim 4, wherein the upper head is made by central flat head and an external toroidal concave head.
 18. A device that enables storing in a vessel a fluid at high temperature, according to claim 4, wherein the upper head is made by central flat head and an external conical section.
 19. A device that enables storing in a vessel a fluid at high temperature, according to claim 18, wherein the upper head is made by central flat head and has an arrangement of flat planes inclined respective to the central axis of the vessel.
 20. A device that enables storing in a vessel a fluid at high temperature, according to claim 4, wherein the upper is head is made by central convex head and an external toroidal concave head.
 21. A device that enables storing in a vessel a fluid at high temperature, according to claim 4, wherein the upper is head is made by central flat head which is connected with a conical section that goes upward, that is connected with an external annular flat horizontal ring to be externally connected to a conical section that goes downward to be connected with the upper ring of the intermediate zone.
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 62. A device that enables storing in a vessel a fluid at high temperature, according to claim 18, wherein the lower zone of the vessel is made by a single concave head.
 63. A device that enables storing in a vessel a fluid at high temperature, according to claim 18, wherein the lower zone of the vessel is made by a ring made by a concave shape that is connected to a cylindrical ring that ends with a concave head at the bottom.
 64. A device that enables storing in a vessel a fluid at high temperature, according to claim 19, wherein the lower zone of the vessel is made by a single concave head.
 65. A device that enables storing in a vessel a fluid at high temperature, according to claim 19, wherein the lower zone of the vessel is made by a ring made by a concave shape that is connected to a cylindrical ring that ends with a concave head at the bottom.
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